With the ever-increasing user demand for a high-speed data service and continuous growth in the number of cell users, the mobile communications network has an ever-increasing demand for spectrum resources. Utilization of a system spectrum of a large-scale multiple-user multiple-input multiple-output (MU-MIMO) system is improved by increasing the number of base station antennas, which gains wide attention. Antennas that are far more than users in number are provided at a base station side of the large-scale MIMO system, while the cell users are provided with an individual antenna. By using the numerous antennas, the base station simultaneously serves multiple terminal users in the same time-and-frequency resource, and obtains uplink and downlink channel estimates of all the users by using a pilot transmitted on the uplink and the channel reciprocity of a Time Division Duplex (TDD) system, thereby achieving downlink precoding.
The large-scale MU-MIMO system is essentially characterized in that the number of the antennas at the base station side increases by more than one order of magnitude in comparison with that of the conventional MU-MIMO system. Compared with the conventional MU-MIMO system, it has the following specific advantages: it achieves higher multiples of capacity, higher power utilization, and higher spectrum utilization; may use a relatively cheap and low-power device; and has better link reliability.
For the conventional large-scale MIMO system, all users in a cell use orthogonal pilots, and the base station performs channel estimation by using these orthogonal pilots and the channel reciprocity of the TDD system, thereby obtaining uplink and downlink channel estimation information of all the users. However, due to coherence time and limitation on the number of users, the same orthogonal pilot sequence needs to be reused in multiple cells, so that the base station is interfered by pilot information sent by users in an intra-frequency cell when receiving uplink pilot information, thereby resulting in pilot contamination.